1. Field of the Invention
The present invention relates to a projection exposure apparatus for use in photolithographic processes or fabricating semiconductor devices (such as LSIs), imaging devices (such as CCDs), liquid crystal displays (LCDs), thin film magnetic heads and others. More particularly, the present invention relates to a scanning exposure type of projection exposure apparatus, such as of a so-called step-and-scan type, in which a mask and a photosensitized substrate are moved for scanning in synchronism with each other and relative to an exposure optical system, so as to serially transfer a pattern formed on the mask onto the photosensitized substrate.
2. Description of the Related Art
The photolithography technique is commonly used in fabrication of semiconductor devices and the like. For this technique, there are used step-and-repeat type of projection exposure apparatuses (such as steppers) in which a pattern formed on a reticle (serving as a mask) is projected through a projection optical system onto each of shot areas defined on a photoresist-coated wafer (or glass plate, etc.). More recently, as larger and larger chips of semiconductor devices are being fabricated, it has become desired to make a projection exposure having a much larger pattern onto a wafer. In order to meet this desire, there has been developed a scanning exposure type of projection exposure apparatus in which a reticle and a wafer are moved for scanning in synchronism with each other and relative to a projection optical system, so that a shot area having a larger extent than the effective exposure field of the projection optical system may be exposed.
One of the known types of projection exposure apparatuses using the scanning exposure technique is the aligner, in which the entire region of the pattern on one reticle is serially projected onto the entire region of the surface of one photosensitized substrate by using a 1:1 projection optical system. More recently, there has been developed a step-and-scan type of projection exposure apparatus, in which each of shot areas on a wafer is exposed by demagnification projection scanning exposure and the exposure site is moved from one shot area to another by stepping the wafer.
In general, the projection exposure apparatus uses a projection optical system which has a large numerical aperture (NA) and thus a very shallow focal depth, so that a certain type of mechanism must be provided for keeping the surface of the wafer coincident with the image plane of the projection optical system so as to enable transfer of a fine circuit pattern with a high resolution.
Therefore, in a conventional one-shot exposure type of projection exposure apparatus, a tilt sensor (or leveling sensor) is provided to measure a tilt angle of the surface of the wafer relative to the guide plane (or slide plane) of the wafer stage. Further, a tilt angle of the image plane of the projection optical system relative to that guide plane is also measured and stored as preparatory data. Then, the tilt angle of the surface of the wafer is controlled by servo-control technique such that the measured value supplied by the tilt sensor may converge to the tilt angle of the image plane and thereby the surface of the wafer is made parallel to the image plane. In addition, by performing such control of the tilt angle (auto-leveling control) together with the so-called auto-focusing control for causing the height of the surface (focusing position) of the wafer to be coincident with the image plane of the projection optical system, the entire region of each shot area on the wafer is kept falling within the range of focal depth about the image plane. So far, the scanning exposure type of projection exposure apparatus also uses essentially the same control technique as that for the one-shot exposure type of projection exposure apparatus as described above, in order to cause the surface of the wafer to be coincident with the image plane.
In the scanning exposure technique, an image of a portion of the pattern on the reticle is projected through the projection optical system onto the wafer and within a slit-like projection exposure area on the wafer (this area is referred to as the "illumination field" hereinafter). In contrast, the area on the wafer within which the entire pattern on the reticle is serially projected, and thus which is larger than the illumination field, is referred to as the "exposure field" (corresponding to a shot area).
In a typical conventional scanning exposure apparatus, and in particular in the demagnification projection step-and-scan type of projection exposure apparatus, the reticle and the wafer are moved for scanning independently of each other and relative to the projection optical system, so that the slide plane of the reticle and that of the wafer are established independently. Further, because it is impossible to achieve the perfect parallelism between the slide plane of the reticle and the pattern surface of the reticle, the level (height) of the image plane of the projection optical system (i.e., the plane in which the image of the pattern on the reticle is formed through the projection optical system) may gradually vary as the position of the reticle in the scanning direction varies. Such gradual variation of the level of the image plane may cause a tracking error if the response speed of the auto-focusing control is relatively low, resulting in that a portion of the surface of the wafer within the exposure field (shot area) may not fall within the range of focal depth relative to the image plane.
Because the auto-focusing (AF) sensor and the tilt sensor are used for detecting the focusing position and the tilt angle, respectively, of that portion of the surface of the wafer which is confined within the projection exposure area (or the slit-like illumination field in the case of the scanning exposure type), the detection areas of these sensors are generally confined within the projection exposure area (or the slit-like illumination field). For this reason, in a conventional control sequence, the focusing and leveling control is performed only during the time period when an exposure area on the wafer is actually exposed. Thus, during the time period when the exposure site moves from one projection exposure area to another, the focusing and leveling control is made inoperative and the positions of the Z-stage and the tilt-stage are kept fixed. Then, when the positioning of the wafer for the new projection exposure area (or the new scanning start position) has been made, the focusing and leveling control is restarted. This control sequence is utilized because the whole path of the movement of the exposure site from one projection exposure area (or the scanning start position) to the next may not necessarily be confined within the regions which are detectable, but may be within the regions which are not detectable, so that it is difficult to continue the focusing leveling control. In the case where the focusing and leveling control is made inoperative during the movement to the next location, the focusing position of the surface of the wafer may significantly vary after such movement if the wafer surface is inclined relative to the guide plane of the wafer stage. This results in large initial adjustments (which equal the focusing error, that is, an amount of defocusing and the tilt angle error) required when the focusing and levelling control restarts at the next illumination field to be exposed, thus a longer settling time (adjustment time) is required for these errors to settle within allowable ranges. This results in a drawback that the throughput of the exposure process (the number of wafers that can be exposed per unit of time) is reduced.
In particular, in a typical step-and-scan type of projection exposure apparatus, the focusing and levelling control of the wafer surface is continuously performed during the scanning exposure on each exposure area while the wafer is moved for scanning. Therefore, if there is on the surface of the wafer a region with a groove-like structure (such as so-called street lines for indicating the border between adjacent semiconductor chips sites on a wafer), which is inappropriate for the region from which measurements are derived for the focusing and levelling control, then the focusing and levelling control may be disturbed, resulting in a lower tracking accuracy as well as a longer settling time.